[TASK] Fix setDBinit regression

Since #34092 has been merged, a missing setting of
$TYPO3_CONF_VARS[SYS][setDBinit] will cause a die.

This change fixes the behaviour and additionally removes the setting
completely (it was in fact removed already but still listed in the
Install Tool).

Change-Id: Ib58311406e4b512cc2f433dab2bf50fba56eaa39
Resolves: #34369
Releases: 4.7
Reviewed-on: http://review.typo3.org/9268
Reviewed-by: Wouter Wolters
Reviewed-by: Felix Kopp
Tested-by: Felix Kopp
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch